Titanium and titanium alloys are important structural materials in the aircraft power plant construction due to their advantageous strength to weight ratio. Such materials are used for components expected to operate at temperatures of up to about 550.degree. C. Use of these components at higher temperatures is desirable, however, not possible due to the oxygen affinity of titanium alloys. Such affinity causes oxygen to diffuse into the surface of titanium alloys and the diffusion begins at about 550.degree. C. to about 600.degree. C., depending on the type of alloy. The results of such oxygen diffusion are surface areas which become brittle due to their having taken up oxygen. As a result, important mechanical characteristics of the titanium alloys are substantially impaired, for example, the vibration endurance limit is undesirably reduced.
An article entitled "Pack-Aluminization of Titanium" by M. F. Galis et al., available at the Titanium Conference in Munich, Federal Republic of Germany, and an article entitled "Oxidation of Aluminide Coatings on Unalloyed Titanium" by F. Streiff et al. and German Patent Publication No. DE-2,153,218 describe coating methods by means of which an economical diffusion coating is possible in order to introduce aluminum into areas close to the surface of the respective structural components.
According to these methods, the structural components to be coated are embedded in a powder packing comprising a filler material, for example, an aluminum oxide, a so-called donor such as aluminum, and a so-called activator such as ALF.sub.3. The donor provides the element to be diffused into the structural component. If the element is aluminum, the donor is, as mentioned, powderized aluminum. The activator has two functions. First, it activates the surface by making it less passive with regard to the diffusion into the structural component to be coated. Second, the activator participates in the transport of the aluminum from the donor to the structural component surface. Once the structural component has been embedded in the powder packing, the packing with the component is introduced into an oven containing a protective gas atmosphere, for example argon, for the annealing treatment. During such treatment aluminum
transported from the donor to the structural surface while halogenides are being produced. The aluminum diffuses into the titanium alloy to form a protective coating. As a rule, such coating comprises the intermetallic phase TiA1.sub.3. The coating or diffusion temperature is within the range of about 800.degree. C. to about 900.degree. C. The coating duration will depend on the desired layer thickness. For typical layer thicknesses within the range of 2 to 50 .mu.m. The duration is about 2 to 20 hours.
The above mentioned known methods have the disadvantage that in spite of the protective gas atmosphere, a small portion of remainder oxygen in the protective atmosphere in the oven or in the powder packing is technically unavoidable. As a result, during the annealing temperature treatment the oxygen causes a so-called oxygen brittleness in the surface zones close to the surface of the structural component being treated. The aluminum containing protective layer, especially as produced according to the above mentioned German Patent Publication (DE) 2,153,218 prevents the taking up of oxygen of the structural component during the operation of such component. However, the protective layer itself is brittle in the same way as a zone close to the surface which became brittle due to taking up oxygen. The protective layer contains mainly the intermetallic phase TiA1.sub.3 but cannot prevent the just mentioned fact that it becomes itself brittle. This fact is problematic because the protective layer adheres to the surface of the structural component without transition so to speak. In other words, there is not gradual diminishing of the aluminum content as it diffuses into the material of the structural component. This jump in the aluminum content entails a respective jump in the material characteristics. For example, the modulus of elasticity, the so-called "E-modulus", and the ductility also have such a jump at the phase boundary between the protective coating and the material of the structural component. Such a characteristic is especially disadvantageous with regard to an incipient crack or fracture behavior of the component. As a result, the finished coated component is protected against oxygen brittleness, however, the coating method itself is an irreversible source of future impairment.
It is further possible that even though there is a jump type change in the alloy composition at the phase boundary between the protective coating and the base material of the component, a more steady or gradual transition appears or is presented due to stray effects in the analyzing method. Thus, such abrupt transition at the phase boundary may even be hard to detect.
Another typical disadvantage of known aluminizing methods is seen in the fact that a continuous decrease in the hardness is present in the direction from the surface inwardly toward the titanium or titanium alloy. This decrease in the hardness is typically not correlated to the concentration profiles of the elements in the coating. Such hardness characteristics which are independent of the content of the metallic elements, are typical for an oxygen embrittlement. The oxygen embrittled zone is located below the diffusion layer and the hardness gradually decreases in the direction toward the core of the structural component in accordance with the decreasing oxygen content. Such a hardness characteristic can also be achieved in that the titanium structural component is annealed in the presence of air without any technical diffusion treatment.